1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a method for selecting cells in multiband system.
2. Description of the Related Art
In general, a European GSM (Global System for Mobile communications) system and a European digital cellular system (DCS) use different frequency bands. That is, the GSM system uses a frequency band of 900MHz and has a GSM cell network, and the DCS system uses a frequency band of 1.9GHz and has a DCS cell network. GSM cells and DCS cells operate independently each other. However, the GSM cell and the DCS cell may exist either independently or overlappedly.
For a pan-European roaming service in which one can be provided with a call service using a GSM phone anywhere in the European Continent, a multiband mobile station is under development, which can provide a call service through the GSM system even in the DCS network. To provide the pan-European mobile communication service, it is necessary to provide a roaming service between the GSM and DCS systems each operating according to different standards. In particular, since the frequency band and cell structure are difference between the two systems, there is required a new cell selecting method performed at an overlapping area of a GSM cell and a DCS cell.
In a single-band system, cell selecting is performed depending on a received signal strength indicator (RSSI). That is, a mobile station detects an RSSI of a service cell (SC) to which the mobile station itself belongs and an RSSI of a neighbor cell (NC) adjacent to the service cell SC, at the boundary of the two cells SC and NC, and selects (i.e., performs a handoff to) the neighbor cell NC when the RSSI of the cell NC is higher than the RSSI of the cell SC. In the single-band system or in the case where the GSM and DCS cells are not overlapped in the multiband system, cell reselecting which depends on the RSSI can be normally performed. However, in the case where the GSM and DCS cells each using different frequency bands are overlapped in the multiband system, cell selecting which depends on the RSSI will have a reduced efficiency. A detailed description of this problem will be made with reference to FIG. 1. FIG. 1 illustrates a cell selecting method in the case where a GSM cell overlaps with a DCS cell.
With reference to FIG. 1, it is assumed that each base station is positioned at the middle of a cell. A mobile station 101 performs communication with a base station in GSM cell 9 at a position 1. In the case where the mobile station 101 travels to position (2), the mobile station 101 is positioned at the overlap area between GSM cell 9 and DCS cell 14. The mobile station selects DCS cell 14 since it is physically closer to the center of DCS cell 14. In the case where the mobile station 101 travels to position 3 into DCS cell 15, the mobile station 101 detects an RSSI transmitted from a base station in DCS cell 15 and an RSSI transmitted from a base station in GSM cell 10 and selects one of the two cells, having the higher RSSI. For the reasons described above, the mobile station 101 selects DCS cell 15 at position 3. However, if the mobile station 101 travels to position 4, the mobile station 101 performs cell reselection even though it has not left the physical boundaries of GSM cell 10, since the mobile station 101 at position 4 is now closer to the center of GSM cell 10 than DSC cell 15. Therefore, each time the mobile station travels to positions 2xe2x86x923xe2x86x924xe2x86x925 starting from the position 1, cell reselecting occurs.
Location registration should be performed at every cell reselecting, causing an increase in load of the base station. In addition, the mobile station should perform unnecessary cell reselecting even in the state where the RSSI is not lower than a threshold, consuming a battery current, which causes a reduction in a battery run-time.
It is, therefore, an object of the present invention to provide a cell selecting method for a multiband system, which prevents unnecessary cell selecting performed at an overlapped area of two cells using different frequency bands.
To achieve the above object, there is provided a cell selecting method for a mobile station in a multiband system using a plurality of frequency bands. The multiband system has independent cell networks for the respective frequency bands, and a memory for storing the number of neighbor cells having a frequency band being different from a frequency band of a service cell and a minimum received signal strength indicator (RSSI) for cell selecting. The method comprises the steps of: detecting RSSIs of the service cell and the neighbor cells and storing the detected RSSIs; determining whether the detected RSSIs satisfy a handoff condition; determining whether a frequency band of the service cell is equal to a frequency band of the neighbor cells, when the handoff condition is satisfied; counting, when the frequency band of the service cell is not equal to the frequency band of the neighbor cells, the number of the neighbor cells having the frequency band being different from the frequency band of the service cell, storing the counted number, comparing the counted number of the neighbor cells with a previously counted number of the neighbor cells, stored in the memory, to determine whether the number of the neighbor cells has increased, and based on the comparison result, determining whether a camping-on condition is satisfied; and camping on the neighbor cell having the different frequency band when the camping-on condition is satisfied, and camping on the present service cell when the camping-on condition is not satisfied.